The present invention relates generally to an internal combustion engine that is started using an integrated starter/alternator induction machine, and more particularly to a method for reducing the starting time and peak phase currents when starting the internal combustion engine.
High performance torque control of induction machines is based on the concept of field orientation. Field orientation controls the stator currents such that independent control of both the flux and the torque in the machine is achieved. The flux angle used for field orientation can be any one of many fluxes present in the machine. For example, the stator, airgap, or rotor flux. Field orientation based on the rotor flux is the most popular.
There are two different strategies for implementing field orientation. The first, Direct Field Orientation (DFO), orients the control of the stator currents to the flux angle obtained from either a measurement of the flux in an induction machine, or from an estimation of the flux. The second, Indirect Field Orientation (IFO), is based on the slip frequency relationship that must be true in order to align the rotor flux on the direct axis.
In the implementation of either DFO or IFO, the stator current in the rotor flux synchronous reference frame is divided into two components. One component is aligned with the rotor flux vector and the other component is perpendicular to the rotor flux vector. The axis that is aligned with the rotor flux is typically called the direct or d-axis. The axis that is perpendicular to the rotor flux is typically called the quadrature or q-axis.
FIG. 1 depicts the reference frames 1 for an induction machine and the relationship between the synchronous (d-q), stationary (x-y) and abc phase windings.
The overall stator current in the induction machine is related to the d and q-axis stator currents by the relationship:
Is2=ids2+iqs2xe2x80x83xe2x80x83(1)
The component of the stator current along the d-axis is solely responsible for the production of the rotor flux:                               λ          dr                =                                            L              m                                                                        (                                                            L                      r                                        /                                          r                      r                                                        )                                ⁢                p                            +              1                                ⁢                      i            ds                                              (        2        )            
The component of the stator current along the q-axis, iqs is proportional to the amount of torque produced:                     T        =                                            3              2                        ·                          P              2                        ·                                          L                m                                            L                r                                              ⁢                      λ            dr                    ⁢                      i            qs                                              (        3        )            
where:
p=number of poles
xcexdr=d-axis rotor flux
ids=d-axis stator current
iqs=q-axis stator current
Lm=magnetizing inductance
Lr=rotor inductance
rr=rotor resistance
Equations (1), (2), and (3) show that independent control of both the rotor flux and torque of the machine can be achieved by controlling the d and q-axis stator currents.
Currently known in the art is a brute force starting method in which the flux current command, ids* and the torque current command, iqs* are both applied at the same time the start command for the internal combustion engine is received. The flux current command, ids* is set to a level that results in a desired steady-state rotor flux level, and remains constant throughout the starting event. The torque current command, iqs* is calculated from the flux current command, ids* and the torque command using equation (3) above subject to the overall stator current limit. FIG. 2 is a graphical representation of the current commands, ids* 2 and iqs* 3, and the resulting torque 4 using the brute force starting method described herein.
There are several disadvantages associated with the brute force starting strategy. First there is a significant delay in the starting response time. The time delay in the starting response is a result of the need to build up the rotor flux in order to produce the desired torque. It is known by equation (2) that there is a low pass filter dynamic on the rotor flux as a function of the flux current command, ids*, with a time constant equal to the rotor time constant, Lr/rr. Therefore, with a constant torque current command, iqs*, the torque also builds up at this rotor time constant.
Another disadvantage is the high peak currents. The high peak currents are a result of commanding a large initial output torque while commanding a large flux current command, ids* to build and maintain the desired rotor flux in the induction machine. During the period of time when the rotor flux is increasing, high stator currents are present in the induction machine in an attempt to produce a large torque. This is in spite of the fact that almost no actual torque is being produced. Therefore, a large percentage of the current, and energy, other than that being used to build the rotor flux, is essentially wasted.
Yet another disadvantage lies in the fact that the high flux current command, ids* is maintained for the duration of the starting event. The result is a further waste of energy. After the initial portion of the start event, little or no torque is required to maintain the engine speed until the engine is started.
In the prior art strategies for starting an internal combustion engine using a starter/alternator induction machine, the peak phase currents are typically very large. Additionally, a delay in the engine start time is caused by starting the engine without any flux in the induction machine and having to wait for the flux to build-up to a sufficient level.
It is an object of the present invention to reduce peak phase currents to reduce the size and cost of the power inverter and not adversely affect the capability of the system. It is another object of the present invention to provide fast response and engine start times by eliminating the delay caused by flux build-up.
It is a further object of the present invention to produce the desired torque as soon as the torque command is received by the system. It is still a further object of the present invention to provide efficient use of the flux energy.
In carrying out the above objects and other objects and features of the present invention, a method is provided for starting an internal combustion engine using a starter/alternator induction machine. i.e. for a hybrid electric vehicle. In the method of the present invention the peak phase currents are minimized, reducing the demand on the power inverter and achieving fast engine start times. According to the method of the present invention, the stator d and q-axis current commands are controlled such that the peak phase currents necessary for starting the internal combustion engine are reduced, while field oriented torque control of the induction machine is maintained, and the torque necessary to provide seamless starting of the engine is produced.
The control method involves pre-fluxing the motor prior to applying the torque command to start the engine. At the time the torque command is applied, the flux command is stepped down to a lower value, allowing the flux to decay to a lower level at a rate equal to the rotor time constant of the induction machine. The control method uses information from vehicle signals and other vehicle subsystems to determine when to perform the pre-flux action.
The method of the present invention is also capable of sensing and determining when a starting event is about to occur. The present invention also provides a strategy for determining the level of starting performance that is required for an impending starting event, as well as how to handle a delayed starting event or a false starting event.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.